1. Field of the Invention
The present invention relates generally to a low-pass filter that is used to attenuate high-frequency components in VHF, UHF, microwave and milliwave bands and, more particularly, to a low-pass filter of the type that has a ground and a signal conductor, such as a coaxial line filter or a stripline filter.
2. Description of the Prior Art
FIG. 1 is a partly exploded, perspective view depicting the structure of a conventional coaxial line filter (a low-pass filter) disclosed in G. L. Matthaei et al., "Microwave Filters, Impedance-Matching Networks, and Coupling Structures," pp.365-374, McGrawHill, 1962. Reference numeral 1 denotes a hollow, cylindrical external ground conductor; 2 denotes a columnar or rod-like signal conductor disposed in the external ground conductor 1 along its axis but spaced apart therefrom; 3 denotes an input terminal connected to one end of the signal conductor 2; 4 denotes an output terminal connected to the other end of the signal conductor 2; 5, 6 and 7 denote disc-shaped, capacitive conductors of the same size which are mounted on the signal conductor 2 concentrically therewith at predetermined intervals in such a manner that the signal conductor 2 extends through the capacitive conductors 5, 6 and 7 at the center thereof; and 8, 9 and 10 denote dielectric rings tightly inserted between the perimeters of the capacitive conductors 5, 6 and 7 and the interior wall of the external ground conductor 1.
The coaxial line filter of the above configuration serves, in its entirety, as an LC ladder circuit wherein those parts of the signal conductor 2 having mounted thereon the capacitive conductors 5, 6 and 7 function as low-impedance lines and the other parts as high-impedance lines.
When supplied at the input terminal 3 with a signal of the VHF, UHF, microwave or milliwave band, the coaxial line filter attenuates a signal component above a cut-off frequency fc determined by the LC ladder circuit, permitting the passage therethrough of a signal component below the cut-off frequency fc for output via the output terminal 4. Thus, the coaxial line filter operates as a low-pass filter.
Because of such a configuration as described above, however, the conventional low-pass filter has some drawbacks; for example, in the case of its multi-stage connection, high-impedance lines of a predetermined electric length produce therebetween resonance at a frequency where the phase of the input signal varies by .pi. for the length of one of the high-impedance lines. As a result, the low-pass filter permits the passage therethrough of signal components of frequencies around resonance.
FIG. 2 is a graph showing the attenuation characteristic of the traditional coaxial line filter. The abscissa and the ordinate represent signal frequency and attenuation value, respectively. Reference character fc denotes the cut-off frequency and fs denotes the resonance frequency of the high-impedance line. As depicted in FIG. 2, the coaxial line filter exhibits a transmission characteristic at the frequency (the resonance frequency fs) corresponding to the electric length of the high-impedance line, resulting in a failure to provide a large attenuation value over a wide frequency band above the cut-off frequency fc.